There are many known load regulating systems, such as employed in systems for converting DC to DC, DC to constant frequency AC, AC to AC and DC to variable frequency AC Many of these converter systems have low output impedance and, therefore, are essentially voltage sources. Voltage source systems usually operate to connect a source of DC voltage intermittently to a load via periodic connections and disconnections established by input and output power switches. The power switches are operated in a predetermined pattern to obtain a desired output wave form, in many instances a stepwise approximation of an ideal wave form for an AC output voltage.
Variable voltage, variable frequency AC output converter systems frequently employ power transistors and/or silicon-controlled-rectifiers to modulate a source to load power flow path. When such converters utilize silicon controlled rectifiers, they must be force commutated, requiring large currents to be diverted from one semiconductor switch path to a second semiconductor switch path in the presence of voltage, to turn off the silicon controlled rectifiers. Power transistors, which can be actively controlled via base electrodes for both turn on and turn off, when used for switches in such converters need not be force commutated from a conducting to a non-conducting state. However, power transistors normally require extensive snubbing, which is usually accomplished with passive circuit elements that dissipate substantial power as the power transistors are activated and deactivated. Silicon controlled rectifiers normally require active auxiliary high power circuit components for force commutating to the cutoff condition.
Load regulation, particularly in converter systems, has been proposed in the prior art by connecting a resonant circuit between a source and load. The resonant circuit is activated typically for an interval equal to one half cycle of the resonant circuit. Thus, half wave sinusoidal pulses are applied via the resonant circuit to the load. In converters operating on this principle, the period of each half sinusoidal current pulse supplied by the resonant circuit to the load is considerably shorter than the period of an AC output wave supplied by the converter to a load.
The prior art resonant circuit systems are relatively complex, employing computers or relatively complex circuitry for controlling the application of the current pulses from the resonant circuit to the load. In one system, a computer selects connections between several input terminals and several output terminals by calculating the connection which results in the least square error between the desired and expected voltages at the output terminals. In another proposed system, connections between several input sources and an output load terminal are changed while current is flowing from one input terminal to the output terminal. This is performed to stabilize the current supplied by the resonant circuit to the load during each application of current from the sources to the load. The change in current is performed by switching between positive and negative voltage sources during each current pulse.
These prior art systems are also characterized, particularly during start up for motor loads, i.e., at low motor speed, by numerous non-allowed states between the several input sources and the output terminals. The numerous non-allowed states can be particularly deleterious on the operation of a motor load because it can cause cogging of the motor, and instability to such an extent that the motor does not run. The number of non-allowed states is great enough in many situations to interrupt the flow of current to the load for an interval that is a substantial fraction of a cycle of the AC voltage driving the motor.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for, and method of regulating a load with, a resonant circuit selectively connected between the load and a source.
Another object of the invention is to provide a new and improved method of, and apparatus for, operating a converter including a series resonant circuit.
Still another object of the invention is to provide a new and improved apparatus for, and method of, supplying power to an AC load with a resonant circuit, wherein the resonant circuit is controlled in a relatively simple manner.
Still another object of the invention is to provide a new and improved apparatus for, and method of, supplying power to an AC load with a resonant circuit having a capacitor that is charged to a level determined by the load voltage.
Still another object of the invention is to provide a new and improved apparatus for, and method of, synthesizing a waveform by means of a resonant circuit connected in series between a source and load across which the synthesized waveform is developed.